Electrophotography has advantages such as excellent instantaneousness and the ability to give high-quality images, and is hence used extensively in the fields of copiers, various printers, and printing machines. As electrophotographic photoreceptors serving as the core of electrophotography, use is being made of electrophotographic photoreceptors (hereinafter, also referred to simply as “photoreceptors”) employing an organic photoconductive material which has advantages such as non-polluting properties, ease of film formation, and ease of production.
Known electrophotographic photoreceptors employing an organic photoconductive material include: a so-called dispersion type single-layer photoreceptor including photoconductive fine particles dispersed in a binder resin; and a multilayer type photoreceptor having superposed layers including a charge-generating layer and a charge-transporting layer. The multilayer type photoreceptor has the following advantages. The multilayer type photoreceptor can be obtained as a high-sensitivity photoreceptor by using a charge-generating material having a high efficiency in combination with a charge-transporting material having a high efficiency. There is a wide choice of materials, and highly safe photoreceptors are hence obtained. Furthermore, since the photosensitive layer can be easily formed by coating fluid application, the multilayer type photoreceptor has high productivity and is advantageous also from the standpoint of cost. For these reasons, photoreceptors of the multilayer type are mainly used, and are being diligently developed and put to practical use.
On the other hand, the single-layer type photoreceptor is slightly inferior in electrical properties to the multilayer type photoreceptor and has a slightly lower degree of freedom of material selection. However, the single-layer type photoreceptor can generate charges in an area near the photoreceptor surface and, hence, can be used to attain higher resolution. Furthermore, even when the photosensitive layer is formed thickly, this does not result in image blurring. The single-layer type photoreceptor hence has an advantage that printing durability can be enhanced by increasing the film thickness. In addition, the single-layer type photoreceptor has an advantage that a cost reduction is possible for the following and other reasons: a smaller number of coating steps suffice for the photoreceptor; this photoreceptor is advantageous for diminishing the interference fringes attributable to conductive bases (substrates) or mitigating pipe defects; and inexpensive bases such as, e.g., pipes which have not been machined, can be used.
Although the electrophotographic photoreceptors employing an organic photoconductive material have the advantages described above, these photoreceptors do not satisfy all the properties required of electrophotographic photoreceptors. Such electrophotographic photoreceptors are desired to be further improved especially in high sensitivity, low residual potential, and durability.
As a result of a growing demand for high-speed printing, there is a desire for materials usable in higher-speed electrophotographic processes. Besides having high sensitivity and a long life, the photoreceptor in this case is required to have satisfactory responsiveness because the time period from exposure to development is shorter. Although the responsiveness of a photoreceptor is governed by the charge-transporting layer, in particular, by the charge-transporting material, it is known that the responsiveness changes considerably with the binder resin.
Many charge-transporting substances of various kinds have been proposed as measures in improving photoreceptor sensitivity, reducing residual potential, and improving responsiveness. For example, patent document 1 includes a statement to the effect that by incorporating a specific charge-transporting substance into a photosensitive layer, the photoreceptor is caused to have high sensitivity, low residual potential, and high mobility.
With respect to durability improvement in photoreceptors, polycarbonate resins have been frequently used hitherto as binder resins for surface layers of electrophotographic photoreceptors. In recent years, however, it has been proposed to use a polyarylate resin, which has higher mechanical strength than the polycarbonate resins, to thereby satisfactorily improve the durability of an electrophotographic photoreceptor (patent document 2).
Furthermore, it is known that by incorporating a polyarylate resin obtained by copolymerizing a diphenyl ether-4,4′-dicarboxylic acid residue having a specific structure with a divalent phenol residue having a specific structure into a photosensitive layer, the photoreceptor is rendered highly excellent in mechanical strength, in particular, wearing resistance (see, for example, patent document 3).    Patent Document 1: JP-A-10-312072    Patent Document 2: JP-A-10-039521    Patent Document 3: JP-A-2006-53549